Casanova, the 18th century lover who used to breakfast on 50 oysters, has been vindicated by a study that proves they really are aphrodisiacs.

And spring, the scientists say, is the time of year the shellfish have their greatest aphrodisiac quality.

The team of American and Italian researchers analysed bivalve molluscs - a group of shellfish that includes oysters - and found they were rich in rare amino acids that trigger increased levels of sex hormones.

The link was announced to 15,000 scientists in San Diego, California, at a meeting of the American Chemical Society last week.

It generated possibly more interest than any other discovery in the society's 126-year history. "I am amazed," said George Fisher, a professor of chemistry at Barry University, Miami, who led the research team with his graduate student Raul Mirza and Antimo D'Aniello, of the Laboratory of Neurobiology in Naples.

"I have been a scientist for 40 years and my research has never generated interest like this.

"For centuries, old wives' tales have said that eating raw molluscs - oysters in particular - would stimulate the libido but there has really been no scientific evidence as to why and if this occurs.

"We think this could be the first scientific evidence of some substance.

"Did Casanova's 50 oysters really make him frisky? Could be." Previous speculation about the powers of oysters has centred on the refuelling powers of their high zinc content.

Zinc is found in sperm and men lose between one and three milligrams per ejaculation.

Dr Fisher and his team, partly funded by the United States National Institutes of Health, bought samples of bivalve molluscs - which also include mussels and clams - from fish markets near Dr D'Aniello's Naples laboratory.

They then used a process called high-performance li-quid chromatography to identify which amino acids were present and in what quantities.

They found two unusual ones - D-aspartic acid (D-Asp) and N-methyl-D-aspartate (NMDA).

"They are not the normal amino acids that Mother Nature uses," said Dr Fisher. "You can't just find them in a vitamin shop."

Dr D'Aniello had found in earlier experiments that injecting the amino acids into rats triggered a chain reaction of hormones that ended with the production of testosterone in males and progesterone in females.

"Increased levels of those hormones in the blood means you are more active sexually," he said.

"Yes, I do think these molluscs are aphrodisiacs. If the male is having difficulties, they have to eat a lot of mussels or oysters.

"Spring, when the molluscs themselves are breeding, is best. There is the highest concentration of these two amino acids then."

He added: "The Italians have been right about this for centuries - since before Casanova, from the time of the Romans."

The scientists stressed that the oysters have to be eaten raw to be most effective. Cooking them reduces the quantity of D-Asp and NDMA molecules.

Casanova, who admitted seducing 122 women in his memoirs, offered his own serving suggestion in Volume Six: "I placed the shell on the edge of her lips and after a good deal of laughing, she sucked in the oyster, which she held between her lips. I instantly recovered it by placing my lips on hers."

In London, at Bibendum, Sir Terence Conran's Chelsea Oyster bar, most customers seem to be taking their oysters in more conventional fashion - with a squirt of lemon, a dash of red wine vinegar and shallots, a drop of tabasco, or just slurped plain from the shell.

"I like the taste," said Claude, 76, a retired university lecturer from Deauville, France, demolishing a dish of Bibendum's fruits de mer with Liliane, his wife. "And no, I won't be eating more now you've showed me the scientific evidence.

"I've already fathered two children and I can assure you that was nothing to do with oysters."

His wife agreed. "Aphrodisiac qualities? It's a myth," she said.

There was similar scepticism from Alex Colas, 35, a university lecturer from Willesden Green, north London, and his wife Ishani Salpadoru, 32, a family doctor.

"I know they have a reputation," said Mr Colas, "but that's not why I am eating them.

"It is a sensual experience though - the fresh taste of the sea, the slippery, silky texture.

"Perhaps that's why it works as an aphrodisiac for some people. If you are find something sensual, it heightens your senses and perhaps one thing leads to another.

"I don't think I get a chemical reaction from them, though."

His wife was trying oysters for the first time. She tipped the shell, slurped and pondered. "There is something sensual about the texture, but no, not sexual," she said.

"I'm probably getting more from the chili sauce, to be honest."

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Maybe I'll go to the local seafood market and get four or five dozen..I can just hear my wife: "What the hell's gotten into you? Get off me already you fat freaking hornbag!" /ccboard/images/graemlins/grin.gif /ccboard/images/graemlins/grin.gif